Circuit for evaluating cells



March'l, 1949. w. E. sco'rT 2,463,321

7 QIRCUIT FOR EVALUATING CELLS Original Filed July 25, 1942 4 Sheets-Sheet 1 IN VEN TOR. 12

ATTORNEY March 1, 1949. w sgoTT CIRCUIT FOR EVALUK'I'ING CELLS OriginalFiled July 25, 1942 4 Sheets-Shes": 2

I INVENTOR. $064M 0 ATTORNEY March 1, 1949. w, sc T 2,463,321

CIRCUIT FOR EVALUATING CELLS Original Filed July 25, 1942 4 Sheets-Sheet3 ATTORNEY March 1,1949.; w. E. seen I 2,463,321

,7 CIRCUIT FOR EVALUATING CELLS ori inal Filed July 25, 1942 4-Sheets-Sheet 4 INVENTORQ- William Sea/Lt ATTORNEY Patented Mar. 1, 1949CIRCUIT FOR EVALUATING CELLS William Edwin Scott, Drexel Hill, Pa.,assignor to E. I. du Pont de Nemours & Company, Wilmmgton, Del., acorporation of Delaware Original application July 25, 1942, Serial No.

Divided and this application March 19, 1946, Serial No. 655,464

Claims. 1

This invention relates to an improvement in means for accuratelyevaluating the output of current sources such as the output ofphotoelectric cells, and it has special significance in goniophotometryfor accurately evaluating the output of photoelectric cells which areresponsive to light reflected from given surfaces. In devices forevaluating reflectance, a suitable light from a col limator may beprojected onto the sample to be tested and the reflected light receivedby a receptor which directs the reflected light onto a measuringphotoelectric cell. The output of this cell may be compared with theoutput of a compensating photoelectric cell which is activated by aconstant or standard of illumination. Thus accurate evaluation of thereflectance depends in part on accurate evaluation of the output of thephotoelectric cells. More particularly, this in vention relates to animproved circuit which is capable of evaluating the output of such cellsaccurately and in proportion to the true reflectance of the sample beingtested.

In a circuit hithereto used for evaluating such conditions ingoniophotometric devices, the photoelectric cells were connected in asimple arrangement of what is known as an opposing parallel arrangement(Fig. 6). The circuit was provided with a zero center galvanometer and aslide wire potentiometer in which the terminals of a measuring cell wereconnected in parallel with the zero center galvanometer and with thefixed ends of a slide wire potentiometer. One terminal of thisphotoelectric cell was connected to the terminal of opposite polarity ofa compensating cell. Then the opposite terminal of compensating cell wasconnected to the variable contact of the slide wire potentiometer suchas the arrangements shown in U. S. P. 2,064,517. A scale indicatingdisplacement or relative position of the variable contact member wassupposed to be proportional to a change in output of the measuring cell,but this contrivance did not give an accurate evaluation because it didnot take into account the small change in resistance of the externalcircuit of the cell in which the output was not changed by the standardof illumination. Thus a small error was introduced into the measurementwhich was material when small differences in output were involved, suchas a change due to the difference in reflectance between a standardsurface and a surface which was tested for relative reflectance. Sucherror might be a large proportion of the difference being estimated.

In order to eliminate errors due to such inherent defects in thedevices, I provide an ex ternal resistance in the measuring cell circuitwhich is varied with variations of resistance in the resistor of theslide wire potentiometer in that circuit. The device is arranged so thatthe resistance in the external resistor is changed in the same amount asthe resistance in the slide wire potentiometer is changed, but when theresistance in one is increased, the resistance in the other is decreasedso that the total resistance remains constant. This variable resistanceelement external to the potentiometer, as will appear in the followingmore detailed description, is a resistance similar to the resistancewithin the potentiometer and in the same circuit. When the potentiometeris adjusted to a new position which is necessary for balancing theoutput of the cells when a new output of one of the cells is aflected byreason of a change in the light reflected from the sample being testedover that reflected from the standard, then the only change indicated bythe displacement of the measuring device is a change which bears a trueproportion to the change in reflectance from the standard and to thechange in output of the cell which receives the same.

The invention will be described with reference to an embodiment of theprinciples involved which are shown and described in my copendingapplication Serial Number 452,325 filed July 1942 (Patent No. 2,406,166,August 20, 1946), of which this is a division. In the drawingsconstituting a part hereof Fig. 1 is a perspective View of agoniophotometer case showing the scales and manipulative devices in thefront side thereof;

Fig. 2 is a View in perspective of the main optical working parts of thegoniophotometer showing the collimator and receptor each with itsphotoelectric cell mounted thereon, both in turn being mounted on anarcuate supporting frame or bed plate which is attached to a framemember having an aperture therein facing the collimator and receptor,and in position for receiving the sample to be tested in properrelationship to these elements;

Fig. 3 is a rear elevational view of the collimator and parts of theadjusting mechanism for the compensating cell and shown with partsbroken away and other parts in section;

Fig. 4 is a cross sectional view taken along the line 4-4 of Fig. 3showing the adjusting mechanism more in detail;

Fig. 5 is a rear elevational view of the receptor and its photoelectriccell with parts broken away;

Fig. 6 is a wiring diagram showing, in general, the type of circuitemployed;

Fig. '7 is a conventional wiring diagram of the circuit showing twophotoelectric cells connected in opposing parallel arrangement with aslide wire potentiometer and an external compensating resistance in thecircuit of one of the cells;

Fig. 8 is a plan view of the panel of the circuit control or decadecontrol unit used in connection with the circuit shown in Fig. '7;

Fig. 9 is a Wiring diagram of the decade resistance unit used foroperating the circuit illustrated;

Fig. 10 is a fragmentary front elevational view View showing details ofthe supports and measuring arrangement.

Referring to Figs. 1 and 2', a base or support member 5 having a plainsurface 6, upon which or against which the sample to be tested is dis-iposed, has an opening or window I therethrough. The portion of thesurface to be tested for reflectance is exposed to the light throughopening 1 in the plane 6 of the base 5, said light being projectedthereon from the collimator 8. The light reflected from the sample isreceived by the receptor 9 which. transmits it in turn to a photoelectric cell l designated herein as the measuring cell. With a givenvalue of incident light projected by the collimator, it is evident thatthe output of the measuring cell ill will vary'in response to thedifferent amounts of light reflected from samples having differentreflectances, and that the variation in the output of the cell It isdependent upon the relative reflectance value of the surfaces tested.The collimator 8 carries photoelectric cell H, which is designatedherein as the compensating cell. This cell receives a given regulatedillumination from a source of light II in a manner hereinafter to bedescribed.

The collimator 8 and the receptor 9 are mounted with their axes radiallydisposed to a point in the surface to be tested and with their axes in aplane parallel to the plane of an arcuate bed plate l2 or track memberand perpendicular to the surface 6. The arcuate bed plate l2 extends onone side of, the frame member in a plane perpendicular to the surface 6of the frame member upon which the surface to be tested is disposed. Themountings for the collimator and receptor are arranged so that theircenters of rotation, as they travel with their axes radially disposedalong the arcuate track, are at a point in the plane it which is withinthe aperture l wherein the sample to be tested is exposed to the light.

The light source H mounted on the bed plate 50 of the collimator isenclosed in a light tight housing [3 (Fig. 3). Over an opening in therear portion of the housing IS, the compensating photoelectric cell IIand a device for regulating the admission of light to the cell from thelight source are mounted. Suitable electrical connections 5| and 52 areprovided for-supplying power to the light source i l. The power supplyto the light source is preferably one which does not vary greatly, butit may be one which is not absolutely constant provided that itmaintainsa propervoltage on the light source corresponding to the desired colortemperature of the light source, such as I. C. I. Illuminant A. Smallvariations in this light, which may occur from day to day in the colortemperature, are not serious. From the following more detaileddescription it will also be observed that both the compensating and themeasuring photoelectric cells are illuminated from one and the samesource, and their outputs are balanced against each other in makingmeasurements of the difference between the reflectance of the sample andthat of a standard. With the arrangement here provided, small variationsin light intensity do not materially affect the null or balance point inmakingreflectance measurements on different days.

The arrangement for supporting the collimator and receptor in properdisposition on the arcuate bed plate l2 provides for restrainingmovement in five directions or points of movement, and permits freedomof movement in one direction. Rollers (Figs. 3 and 4) on the arms Itwhich are attached to the closed housings of the collimator and receptorroll in a flanged arcuate guide or track ll which is integral with theoutside edge of bed l2. Supports H3 at the inner ends of the collimatorand receptor mountings are adapted to slide on a semicircular extensionor table 20 which comprises an integral part of the frame or base member5. The sliding contact surface-of table 26 is perpendicular to thefacing 6 of base member 5 and it extends from the same side of. the basemember as the arcuate track 2; The

. ends of springs 2i hook or attach to parts 22 of the base 5 and theother ends attach to parts-.23 and 24 of the collimator and receptorsupports for holding these members on and against the support 23 and thearcuate track l1. Thus arcuate movement of the collimator and receptoralong the track and about their common center of rotation is permittedWhilst the radial disposition of the parts is maintained. Spring guides25,- which are mounted on the collimator and receptor supports 58, arearranged to bear outwardly on the. upper side of the arcuate flange llas additional guides to prevent accidental or unintended movement of thereceptor and collimator when these parts are not locked in place.

Thus the collimator and receptor arms may move in an arc in a planeperpendicular to the plane 6 of the support 5, the center of which islocated in the exposed face of the sample to be tested which extendsacross the aperture? of the frame 5. The coil springs 21 urge each ofthe rollers H5 in a direction such that they havea can tripetal, as wellas a lateral pressure on the track Hand its outside radial flange. Therollers therefore provide four of the five guiding contacts, the fifthbeing between the extensions N3 of the collimator and receptor and theinner sliding-support or table 20.

In mounting the device in the casing 30 (Fig. 1), the bed plate 5 issecured to the upper part of the casing. The top of the casing and thesurface 6 of the bed plate 5 are substantially flush with each other.The aperture 1 through the sample supporting portion of the frame 5permits the light projected from the collimator. 8.to illuminate thesample and to be reflected into the receptor 8.

A sample holder on the casing 38 may be provided. This holder comprisesan angle piece or bracket 3| attached to the back member 22.0f the framemember 5 by screws 32. A yieldable spring steel strip 33 supports ablock 34 to which the projecting fingers of the spring 33 are connectedby rivets M. A knurled knob 35 secured to the top of the block 34servesas a finger grip for raising the block against the spring 33 to mountthe sample. For convenience, the lower surface of the block 35 may befitted with a piece of black plate glass 3B-or other suitable materialto be used. as a standard for. relative; measurements and the sample tobe tested may be inserted underneath the glass plate.

Hinged doors 31 and 38 in the front wall of casing 38 provide access tothe adjusting knobs 40 (Figs. 3 and 4) and other adjusting mechanismsmounted on the end of the collimator. A large door, not shown, isprovided in the rear wall of the casing 39 for providing accessgenerally to the interior of the casing.

Through the front panel of the housing 3&1, an arcuate slot ll! isprovided in which is presented an arcuate plate 4! (Fig. which is markedoff in equal divisions. The plate 4! is attached to the front of thearcuate base member [2. Small Vernier plates d2 concentric with thearcuate plate 41 which operate with and are attached to the collimatorand receptor mountings, respectively, are also visible and accessiblethrough the arcuate slot it. The vernier plates 42 carry posts 43 (Figs.1 and 10) terminating in knobs 44'. For approximate angular adjustmentof the collimator and receptor, the arms 44 of the collimator andreceptor supports and the attached receptor and collimator are moved tovarious positions on the track H by means of the knobs M which projectoutside of the housing. Vernier screws 45 are threaded through blocks 46and into posts it, and locking screws 5'. threaded in and passingthrough blocks AG bear on the front facing of arcuate bed l2 for lockingthe collimator and receptor carriages when the collimator and receptorare placed approximately accurate angular disposition to each other.These Vernier screws r operate on blocks li and posts 43 to retain theparts in the desired location. The parts of these clamping and adjustingdevices are arranged in reverse position to each other on the collimatorand receptor carriages; that is, they are arranged as mirror images ofeach other.

The clamping device comprises a back piece 45' (Fig. 10) which carriesthe guide springs 25 bearing on the flange on the back side of the bed12 and it extends around the inner arcuate edge of bed 12 where theplate 45' is attached by screws 4?. Plate 45 extends along the frontside of the arcuate plate 52. The post or block 16 is mounted integrallywith the plate lii and a locking screw 4'! is threaded in the block iiiand in locking position it bears at its inner end on the bed plate l2.The Vernier screw 45 is also thread ably mounted in the block 55 in atransverse direction to the locking screw 41 and it is rotatably securedat the end into the block 13 which is integrally mounted on the Vernierscale 42. In operation, the collimator and receptor are roughly adjustedto the proper angular disposition by means of the knob 44. The lockingscrew H is then tightened onto the bed plate l2. Thereafter accurateangular adjustment is secured by manipulating the Vernier screw A5. Thecollimator frame 59 carries the lamp N (Fig. 3) and suitable leads 5|and 52 are connected to a suitable source of power. Over an opening inthe rear part of the housing 53 of the collimator is mounted theassembly comprising the compen sating cell I l and the cover plate 23.The latter is suitably secured to the housing by thumb screws 52.

The mounting for the cell H comprises a means for adjusting the amountof light admitted directly to the compensating cell from the source oflight l l. The adjusting means comprises a housing 55 carrying anapertured plate 56 which is attached to the inner surface thereof by thescrews 51. The plate 56 has a circular opening 58 which is centered inthe line extending from the source of light and normal to the face ofthe photoelectric cell ll. Circular openings 59 of various sizes andsmaller than opening 58 are centered between the plate 56 and the cellll along the same line as opening 58, by mounting a rotatable disc 68 onthe frame 55 by means of I the screw 6| so that the various sizes ofopenings may be brought between the large circular aperture 5B and thephotoelectric cell it by rotating the disc on the central or axial screw6t. Interposed in the casing 55 between the rotatable disc t!) and thecell H is a heat absorbing unit 62 which may desirably be composed of ablue heat absorbing glass. Interposed between the heat absorbing glass62 and cell i l is a suitable diffusing screen 63 which may be a pieceof ground plate glass mounted normal to the line passing through theapertures. In order further to regulate the light admitted to thecompensating cell H, a slide 64 is provided in the space between theground glass 63 and the heat absorber 52. This regulator comprises a bedplate 65 fixed to the frame 55 by screws 6%. The bed plate has arectangular opening 61 therethrough. The slide 64 is preferably providedwith a diagonally extending edge for masking the opening 6'! in the bedplate 55 as the slide is advanced across the opening by means of theadjusting screw 40 which is mounted on the frame 55 and threadablyengages the slide.

In operation, the direct light shed from the light source ll upon thecompensating cell can be cut down to a value such that the output of thecompensating cell balances the output of the measuring cell It. Thecompensating cell Ill on the receptor is activated by the lightrefiected from a standard reflectance piece to which the sample to betested is referred. The output of the compensating cell i! is firstbalanced against the output of the measuring cell iii when the standardof reflectance, say a black piece of plate glass, is mounted in theaperture 7. The balancing of the circuit is done by manipulating thedecade resistance hereinafter to be described. The surface to be testedfor reflectance is then substituted for the standard of reflectance andthe outputs of the cells are again balanced. By noting the difierence inadjustment indicated on the resistance unit when these different outputsof the cells are balanced, an evaluation of the difference inreflectance is shown between the sample and the standard to which thepiece to be tested is referred.

The receptor 9 (Fig. 5) is also provided with light regulating means infront of the measuring cell l8 which is located at the rear of thereceptor. At the focal point of the receptor collimator lens if! is anadjustable plate it having a slit l2 which is disposed at the focalpoint of the receptor collector lens. The plate ll extends diametricallyacross the end of the receptor 2. The plate H is arranged to slide in aframe '13 which is mounted on the end of the receptor 2 by means ofscrews M. The slit 72 in slide ii is properly positioned by means of anadjustable spring press pin it which bears against an end plate on thebracket ll which is an integral projection of the housing. Between theplate H and the measuring cell it, a ground glass Til is mounted normalto the axis of the receptor for disseminating the light which passesthrough the slit. The photoelectric cell iii is then mounted at the endof the housing and held in place by brackets 19'and with its face normaltothe axis side of this cell is connected to post I through connector88. By manipulating the knob A on the panel of the decade resistanceunit, the movable contactor puts more or less of the resistor 88 intothis circuit as connector point 81 (Fig. 7) moves along the resistor 9|.The resistor is proportioned so that this variation amounts to ohms foreach position of knob A. Unit resistances are put into the circuit bymanipulating knob B on the panel, whereby more or less of the resistor9! is entered into the compensating cell circuit as contact point 8!moves along the resistor. The changes in position of contactor points 81and 81 do not affect the resistance in the measuring cell circuit inwhich the resistors 88 and 9| are in series. However, in order toprovide for a third graduation, e. g. a one-tenth ohm variation in thecompensating cell circuit, I provide a resistor 99 in the compensatingcell circuitin series with and interposed between theresistances 88. and9|. This third resistor 90- is varied by moving the contact point 89along theresistor. The variation in this resistance is'made bymanipulating knob C of the panel and its corresponding movable contact89. varies the resistance in both cells circuits. In order, however, toretain the resistance at constant value in the measuring cell circuit, Iprovide the resistor E which is like resistor 99 but protect thegalvanometer. "For this purpose, the

resistor 95 is permanently connected in series with the galvanometer Gin all three connections which are made by the switch D (indicated inFig. 8 as shunt, adjust and read). These three positions of the switch Dclose various circuits through contact points 96, 91 and 98,respectively. In the shunt position of switch D, the circuit from line83 comprises binding post 4, resistor 95, contact point 99, switch D,contact point 96 and the shunt resistor I90 around the galvanometer G.Thence the circuit of the measuring cell is closed through contact point89 and includes resistors E, 9|, 99, contact point 39 and resistor 88.In the adjust position of the switch. D the contact point 91 iscontacted instead of contact point 98 and resistor IN is substituted inthe circuit for resistor Hill. In the read position of the switch D, adifierent kind. of connection is made. In this position, switch D makescontact with contact point 98 and brings resistor W2 between the switchD and the line so that it is in parallel with resistor 95' and in serieswith the galvanometer G. Resistor I02 is a very small value ofresistance and this connection gives the minimum of protection andgreater sensitivity of reading to the galvanometer. Still greatersensitivity can be introduced by closing This movement an auxiliaryswitch I83, thereby formingaconnection through line I04 without.insertedfre sistance between the line and galvanometer, said connectionbeing in parallel with resistors and I92. the panel arrangement (Fig. 9)and itmay be. located on the panel upon which switch D is mounted at anyconvenient point. Suggestivev relative values for the resistors 95, I80,Illl and I82 are. indicated in Fig. 9. These values are suggested foruse with a zero center D. 0. high. sensitivity galvanometer having aresistance of 40 ohms, a sensitivity per millimeter division of 0.01microampere, and an external critical.

damping resistance of 225 ohms. Such apparratus is suitable for use withbarrier layer type photoelectric cells, such as those comprisingselenium (on iron) barrier construction.

In making a measurement of light distribution. with the parts of thedevice in proper adjustment, a suitable standard, say a black polishedplate glass, is placed in position over the aperture. 1. The decadedials are then set .at the. values. thereof which are selected torepresent the standard. With the receptor and collimator set at theoptimum angle with-respect to the. surface being tested, as indicated bythe arcuate scales in the front of the casing 30, the shutter over thecompensating cell is adjusted to give. a zero deflection on thegalvanometer G. The value of reflectance given to the. glass plate maybe arbitrary, or it may be expressed in terms of some hypotheticalstandard, such as a perfect mirror.

The receptor arm is then set at various angles, from a position adjacentto the collimator arm, to a position as iar removed as possible vfromit. The steps or angular positions, which can readily be determinedempirically, should be sufficiently small to furnish adequate data fromwhich an accurate angular distribution curve may be plotted. At each ofthe receptor settings, the decade dials are readjusted to bring thegalva nometer to zero. Depending upon the purpose of 7 the operator, thewhole or only a portion of the distribution curve may be determined inthis way. The resultant readings of the dials are used in plotting thedistribution curve of the reflected light.

For a relative brightness measurement the standard setting for thecollimator is 45 and for the receptor 0 or vice versa. This measurementmay be made by replacing the standard black glass with a plain magnesiumoxide sur-. face or other standard. The receptor slide may be removedwhen making this measurement in order to admit more light from thereceptor to the measuring cell. This greatly increases the sensitivityof the instrument in making this measurement for angles close to thenormal (when the receptor or collimator is set at or near thisposition), without impairing the accuracy of. the instrument, since itprovides a. more uniform distribution of reflected light for angles.closeto the normal. However, the aperture is desirably suflicientlyrestricted even at this angular disposition of the parts to preclude themeasurement of specularly reflected light. For opaque materials, asuggested standard setting for the collimator is 45 and for the receptor0.

The decade dials are then set at the values thereof which are selectedto represent the standard of relative brightness, and the galvanometerreading is then brought to zero by adjusting the shutters over thecompensating photo This auxiliary switch I03 is indicated-.111

electric cell in the manner heretofore described. With the instrumentthus set for the standard of relative brightness, this standard sampleis replaced by the sample to be compared therewith, and the galvanometerreading is again brought to zero by manipulation of the decaderesistance unit. The difference between the readings of the lattersetting of the dials and those obtained with the standard in placerepresents the difference in relative brightness. From a knowledge ofthe absolute brightness of the standard, these values can be convertedto absolute brightness by an obvious simple calculation.

By variously manipulating the instrument and by variously equipping thereceptor with suitable arrangements between the rear of the receptor andthe measuring cell, a variety of light measurements or comparisons withgiven standards can be made and given relative numerical values by meansof the described balancing device, such as measurements for hiding powerand color measurements of transparent or opaque materials.

It is further evident that the balancing device is useful for balancingthe outputs of other current output devices besides photoelectric cells,and since it is evident that widely different embodiments of thisinvention may be made Without departing from the spirit and scopethereof, it is to be understood that the invention is not limited to thespecific embodiments thereof which are hereinbefore set forth, except asdefined in the appended claims.

I claim:

1. A device comprising a compensating photoelectric cell having itsoutput terminals connected in a circuit comprising three resistanceelements which are connected in series, independently movable contactmembers in said circuit contacting the end members and bridging theintermediate resistance element of said series of resistance elementsfor varying the resistance in said circuit, a measuring photoelectriccell, a circuit connecting the terminals of said cell in which theoutput terminals of said measuring cell are connected oppositely to theterminals of said compensating cell, said measuring cell circuitincluding the three resistance elements of said compensating cellcircuit and a fourth resistance element external to said first namedresistances, all four of said resistances being connected in series insaid measuring cell circuit, a contact member in said measuring cellcircuit movable along said intermediate resistance element for varyingthe resistance in the circuit, another movable contact member in saidmeasuring cell circuit contacting along said fourth resistance elementfor varying the resistance in the same circuit, and meansinterconnecting said last named variable contact members for disposingone member which contacts with said fourth resistance element so thatthe resistance in the circuit will be varied in opposite effect as theresistance in said measuring cell circuit is varied by the movement ofthe other contact member which is in contact with said intermediateresistance.

2. A device in accordance with claim 1 which comprises a galvanometerand connections in said measuring photoelectric cell circuit in parallelwith said measuring cell, said connection being made between said celland said four resistance elements.

3. A device in accordance with claim 1 which comprises a galvanometerand connections in said measuring circuit in parallel with saidmeasuring photoelectric cell, said connections being between said celland said four resistance elements, said galvanometer connectionscomprising a permanently connected resistor in series with thegalvanometer and in parallel with said photoelectric cell, a secondresistor, means for connecting said second resistor in the galvanometercircuit in parallel with the galvanometer and in series with the firstsaid resistor, another resistor, means for substituting the same forsaid second resistor in parallel with the first resistor and in serieswith the galvanometer.

i. A device in accordance with claim 1 which comprises a galvanometerand connections in said measuring photoelectric cell circuit in parallelwith the measuring photoelectric cell, said connections being betweensaid cell and said four resistance elements, said galvanometerconnections comprising a permanently connected resistor in series withthe galvanometer and in parallel with the cell, and means for connectingthe galvanometer directly across said measuring cell circuit.

5. A circuit comprising a compensating photoelectric cell disposed toreecive light directly from a light source of constant value, ameasuring photoelectric cell disposed to receive a variable quantity oflight, output terminals on said cells, conductors including twocompensating variable resistances and a third variable resistance inseries with said compensating cell, conductors including all of saidresistances and a further variable resistance in series with saidmeasuring cell, the output of said measuring cell being opposed to theoutput of said compensating cell, means for varying the resistance inthe coinpensating cell circuit for balancing the compensating cellagainst the measuring cell when the output of said measuring cell isvaried from a predetermined standard value by a variation in receptedlight transmitted to the cell, and means for varying said fourthresistance inversely as the resistance in said third resistance isvaried for maintaining the total resistance in the external circuit ofthe measuring cell at a constant value.

WILLIAM E. SCOTT.

REFERENCES CITED The following references are of record in the file ofthis patent:

Certificate of Correction Patent No. 2,463,321. March 1, 1949.

WILLIAM EDWIN SCOTT It is hereby certified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows:

Column 3, line 14, strike out the word View; column 4, line 16, beforethe word edge insert farcuate; column 7, line 17, for point 87 readpoint 87; line 40, after out insert 0 and that the said Letters Patentshould be read with these corrections therein that the same may conformto the record of the case in the Patent Ofiice.

Signed and sealed this 5th day of July, A. D. 1949.

THOMAS F. MURPHY,

Assistant G'ommz'ssz'oner of Patents.

